The present invention relates to a cathode material of a lithium secondary battery, particularly a LiNiO2 type layered compound having high capacity and excellent cycle and safety properties.
Since a lithium secondary battery has a high energy density as compared with conventional secondary batteries, it has been widely used as a battery for electronic appliances such as mobile phones, portable video cameras, notebook type personal computers, and the like, and in the future, it is expected to be used as a dispersion installation type power source for use in households and electric vehicles. Therefore, investigation and development is currently being undertaken to obtain a battery with improved capacity and efficiency.
A cathode active material presently being sold on the market for a lithium secondary battery is LiCoO2. However, its thermal stability is insufficient and the reserves of cobalt are limited. Hence, this compound has disadvantages in terms of stable supply and cost.
As a substituent for LiCoO2, an LiMn2O4 type spinel compound has drawn attention owing to abundant reserves and cost advantages.
Although an LiMn2O4 type spinel compound has excellent thermal stability and safety properties, it has a capacity per unit weight (hereinafter referred to as active material capacity) of only about ⅔ of that of the cobalt compound and therefore, is scarcely used for mobile phones or the like which require a high capacity.
Furthermore, a secondary battery produced using the spinel compound has an insufficient cycle property and an inadequate self-discharging property at a temperature of 50xc2x0 C. or higher. In addition, there are serious problems in practical use of the battery in electric vehicles, in which the battery is most highly expected to be utilized.
Thus, enthusiastic investigation has been performed for a nickel-cobalt compounded oxide whose reserves are estimated high and whose active material capacity exceeds that of a cobalt-based compound.
However, such a compound is difficult to synthesize in atmospheric air and requires an oxygen atmosphere, and Ni of the compound tends to occupy the Li site. Thus, there are many technical problems to overcome in order to produce by a practical method a compound having sufficiently satisfactory properties.
Furthermore, such a compound has the problem that both the cycle property and the thermal stability are inferior to those of LiCoO2.
For purposes of improving these defects, additional elements have been added to the nickel-cobalt compound.
Japanese Patent Laid-Open No. 2000-90933 and Japanese Patent Laid-Open No. 10-134811 provide a discussion of the selection of elements to be added to such a compound. According to these references, elements having a size 0.8 to 1.5 times as large as the ion radius of Ni3+, 0.56 xc3x85, are stated as being proper.
J. Power Sources 81-82 (1999) 416-419 discloses improving cycle property by adding Mn. However, this results in providing an active material capacity which is decreased to 150 mAh/g. Since the active material capacity of the cobalt-based compound is 150 mAh/g, the advantage of the nickel-cobalt-based compound is canceled.
J. Power Sources 81-82 (1999) 599-603 discloses cycle property improvement by adding F which results in providing about 180 mAh/g active material capacity. However, no discussion of thermal stability is given.
J. Power Sources 68 (1997) 131-134 reports that addition of aluminum to nickel leads to an improvement of thermal stability. However, in this case, the active material capacity is also decreased to about 150 mAh/g.
The active material capacity is decreased by the addition of other elements to the nickel-cobalt compound because the absolute amount of nickel is decreased.
The addition of a single element requires a relatively large added quantity to improve either the cycle property or the thermal stability. This relatively large addition inevitably decreases the active material capacity. To overcome such disadvantages, two or more kinds of elements have to be added.
Addition of two or more elements also has been disclosed, for instance, in Japanese Patent No. 3045998 which discloses that both the cycle property and the thermal stability are improved by adding titanium and magnesium. However, in this case, the active material capacity is 160 mAh/g or lower.
In Japanese Patent Laid-Open No. 2000-113890, the cycle property is increased by the addition of aluminum and magnesium; however, the active material capacity is 150 mAh/g or lower in this example.
Thus, the fact that a decrease in active material capacity cannot be suppressed even if two kinds of elements are added is attributed at least partially to the selection of improper elements, and it is also believed that it is difficult to improve all of the properties by combination of two kinds of elements.
Addition of three or more elements is disclosed in Japanese Patent Laid-Open No. 10-241691. Mg is used as an essential element to add to improve the properties of cobalt-based, nickel-based, and manganese-based materials, and the patent discloses that Mg contributes to improvement of electron conductivity and cycle property.
In this case, although there is given a description that the thermal stability may also be improved, the effect on the active material capacity is uncertain. Further, Japanese Patent No. 3088716 discloses that the addition of Mg does not improve electron conductivity and cycle property.
Thus, it is clear that the types and numbers of elements to be added are very difficult to determine, and available opinions on the subject from various laboratories, institutes and the prior art are thoroughly different.
An object of the present invention is to solve the above-mentioned problems and to provide a method for producing a cathode material having a high capacity, an excellent cycle property, and a safety property by adding three or more kinds of elements to a lithium nickelcobaltate.
The inventors of the present invention have found that an addition of three or more kinds of elements selected from Ti, Mg, B, and Al provides remarkable improvements of properties as results of extensive investigation of a method for producing LixN1-aCoaO2 carried out to achieve the foregoing purposes.
Furthermore, in the case of producing the foregoing material, the inventors of the present invention have determined that: the elements to be added must be mixed evenly; a coprecipitation method provides a proper method to satisfy this requirement; and with respect to the reaction with an Li compound, a firing method must be carried out in a proper manner.
The present invention has been developed based on the foregoing findings and provides a cathode material for a lithium secondary battery having a high capacity, an excellent cycle life, and thermal stability.
According to the present invention, the cathode material for a lithium secondary battery is a layered compound having a general formula of LixNi1-a-b-c-dCOaM1bM2cM3dO2 of which M1, M2 and M3 are selected from the group consisting of Ti, Mg, B, and Al. In the formula, the reference characters x, a, b, c and d satisfy the following: 1.0xe2x89xa6xxe2x89xa61.2; 0.1xe2x89xa6axe2x89xa60.3; 0.005xe2x89xa6bxe2x89xa60.1; 0.005xe2x89xa6cxe2x89xa60.1; 0.005xe2x89xa6dxe2x89xa60.1; and 0.115xe2x89xa6a+b+c+dxe2x89xa60.4.
According to another aspect of the present invention, a method for producing the above described cathode material for a lithium secondary battery is provided. To this end, the above described material is produced by mixing Ni1-a-b-c-dCoaM1bM2cM3d(OH)2 produced by a coprecipitation method with an Li compound and by firing the resultant mixture at 480 to 850xc2x0 C. in the atmospheric air or in an oxygen atmosphere.
According to a further aspect of the present invention, a method for producing the above described cathode material for a lithium secondary battery is provided in which the above described material is produced by mixing Ni1-a-b-c-dCOaM1bM2cM3d(OH)2 produced by a coprecipitation method with an Li compound, by firing the resultant mixture at 480 to 630xc2x0 C. for 15 to 40 hours in the atmospheric air or in an oxygen atmosphere, by pulverizing the obtained material, and by further firing the pulverized material at 700 to 850xc2x0 C. for 3 to 10 hours in the same atmosphere.